The solar gravitational redshift from HARPS-LFC Moon spectra. A test of the General Theory of Relativity

Abstract

The General Theory of Relativity predicts the redshift of spectral lines in the solar photosphere, as a consequence of the gravitational potential of the Sun. This effect can be measured from a solar disk-integrated flux spectrum of the Sun's reflected light on solar system bodies. The laser frequency comb (LFC) calibration system attached to the HARPS spectrograph offers the possibility to perform an accurate measurement of the solar gravitational redshift (GRS) by observing the Moon or other solar system bodies. We have analysed the line shift observed in Fe absorption lines from five high-quality HARPS-LFC spectra of the Moon. We select an initial sample of 326 photospheric Fe lines in the spectral range 476-585 nm and measure their line positions and equivalent widths (EWs). Accurate line shifts are derived from the wavelength position of the core of the lines compared with the laboratory wavelengths. We fit the observed spectral Fe lines using CO${}^5$BOLD 3D synthetic profiles. Convective motions in the solar photosphere do not affect the line cores of Fe lines stronger than about $\sim 150$ mA. In our sample, only 15 FeI lines have EWs in the range $150<$ EW(mA) $<550$, providing a measurement of the solar GRS at $639\pm 14$ $\mathrm{m}{\mathrm{s}}^{-1}$, consistent with the expected theoretical value on Earth of $\sim 633.1$ $\mathrm{m}{\mathrm{s}}^{-1}$. A final sample of about 97 weak Fe lines with EW $<180$ mA allows us to derive a mean global line shift of $638\pm 6$ $\mathrm{m}{\mathrm{s}}^{-1}$ in agreement with the theoretical solar GRS. These are the most accurate measurements of the solar GRS so far. Ultrastable spectrographs calibrated with the LFC over a larger spectral range, such as HARPS or ESPRESSO, together with a further improvement on the laboratory wavelengths, could provide a more robust measurement of the solar GRS and further tests for the 3D hydrodynamical models.

arXiv commentAccepted for publication in Astronomy & Astrophysics

References in corpus (30)

• Solar Chemical Abundances Determined with a CO5BOLD 3D Model Atmosphere
• Nominal values for selected solar and planetary quantities: IAU 2015 Resolution B3
• Simulations of stellar convection with CO5BOLD
• A new code for automatic determination of equivalent widths: Automatic Routine for line Equivalent widths in stellar Spectra (ARES)
• The HARPS search for Earth-like planets in the habitable zone: I -- Very low-mass planets around HD20794, HD85512 and HD192310
• Using the Sun to estimate Earth-like planets detection capabilities II. Impact of plages
• Planetary detection limits taking into account stellar noise. I. Observational strategies to reduce stellar oscillation and granulation effects
• The Sun as a planet-host star: Proxies from SDO images for HARPS radial-velocity variations
• Using the Sun to estimate Earth-like planets detection capabilities.I. Impact of cold spots
• Instrumentation for the detection and characterization of exoplanets
• The IAG solar flux atlas: Accurate wavelengths and absolute convective blueshift in standard solar spectra
• How realistic are solar model atmospheres?
• Non-LTE line formation of Fe in late-type stars IV: Modelling of the solar centre-to-limb variation in 3D
• Reconstructing the solar integrated radial velocity using MDI/SOHO
• The spectrum of Fe II
• A Frequency Comb calibrated Solar Atlas
• Fe I line shifts in the optical spectrum of the Sun
• A crucial test for astronomical spectrograph calibration with frequency combs
• Long-term radial-velocity variations of the Sun as a star: The HARPS view
• 3-D non-LTE radiative transfer effects in Fe I lines: II. Line formation in 3-D radiation hydrodynamic simulations
• A catalogue of accurate wavelengths in the optical spectrum of the Sun
• Impact of granulation effects on the use of Balmer lines as temperature indicators
• Stellar Surface Magneto-Convection as a Source of Astrophysical Noise. I. Multi-component Parameterisation of Absorption Line Profiles
• Tool for Automatic Measurement of Equivalent width (TAME)
• Precision and consistency of astrocombs
• Wavelengths of the 3d6(5D)4s a6D - 3d6(5D)4p y6P Multiplet of Fe II (UV 8)
• The MarkI helioseismic experiment.I. Measurements of the solar gravitational redshift (1976-2013)
• ESPRESSO on VLT: An Instrument for Exoplanet Research
• Detection of Gravitational Redshift on the Solar Disk by Using Iodine-Cell Technique
• 3D spectral synthesis and rotational line broadening

Cited by in corpus (18)

• Rapid contraction of giant planets orbiting the 20 million-years old star V1298 Tau
• Thermal-light heterodyne spectroscopy with frequency comb calibration
• Non-detection of ${}^6$Li in Spite plateau stars with ESPRESSO
• A limit on variations in the fine-structure constant from spectra of nearby Sun-like stars
• Spatially resolved spectroscopy across stellar surfaces. IV. F, G, & K-stars: Synthetic 3D spectra at hyper-high resolution
• Convective characteristics of Fe I lines across the solar disc
• Solar center-to-limb variation in Rossiter-McLaughlin and exoplanet transmission spectroscopy
• Testing Einstein's Equivalence Principle and its Cosmological Evolution from Quasar Gravitational Redshifts
• Cosmological test of local position invariance from the asymmetric galaxy clustering
• The solar photospheric silicon abundance according to CO5BOLD: Investigating line broadening, magnetic fields, and model effects
• Improved precision of radial velocity measurements after correction for telluric absorption
• A Measurement of Stellar Surface Gravity Hidden in Radial Velocity Differences of Co-moving Stars
• Detection of Gravitational Redshift in Open Cluster non-degenerate stars
• Estimating the strength of Lorentzian distribution in non-commutative geometry by solar system tests
• Asymptotically Schwarzschild solutions in $f(R)$ extension of general relativity
• 3D non-LTE Ca II line formation in metal-poor FGK stars. I. Abundance corrections, radial velocity corrections, and synthetic spectra
• Radial velocities: direct application of Pierre Connes' shift finding algorithm to Cross-Correlation Functions
• Detecting Lorentz-violation induced by a tensor field with S-star's motion around Sgr A*